Looking inside molecular bonds at biological interfaces with dynamic force spectroscopy

Weak non-covalent interactions between large molecules govern interfacial structure and adhesion in biology. Because of thermal activation, these bonds have modest lifetimes and bond lifetimes are progressively shortened under application of external force. Theory predicts that bond survival time depends on how fast the force is applied and the expected survival time specifies the most likely breakage force (strength) at a given loading rate (force/time). Plotted as a function of log(e) (loading rate), the dynamic spectrum of bond strength provides an image of the prominent barriers traversed in the energy landscape along the unbinding pathway, which establishes a direct link between measurements of bond force and molecular-scale chemistry. Experimentally, the challenge is to measure bond strength over several orders of magnitude in loading rate. With a recently designed probe technique, we have measured strengths of single receptor-ligand bonds and receptor-membrane anchoring over an enormous range of loading rates from 10(-1) pN/s to 10(5) pN/s, which reveals an inner view of the complexity of these interactions.